This invention relates in general to electrostatic copying systems, and in particular, to an improvement in the transferring of a toner pattern produced by an electrostatic image.
When a toner pattern produced by an electrostatic image is to be transferred from one surface to another, a charge is delivered to the surface to which the image is to be transferred.
This charge, being of an opposite polarity as compared to the toner charge, causes the toner particles to be attracted to the desired surface.
In the prior art, the charge delivered is in the form of an electrical potential established between the surface of the transfer member and the surface bearing the toner image. The efficiency of the transfer is directly related to the value of the electrical potential. However, there is a maximum value for the potential above which breakdown of the transfer surface may occur as a result of the transfer member acting as a dielectric for a "capacitor"in which the transfer device and the photoconductive backing member are the plates. At breakdown the dielectric no longer isolates the plates but allows a disruptive discharge to occur. This disruptive discharge between the image surface and the transfer surface results in at least partial destruction of the toner image. The maximum value depends on the transfer material and a number of environmental factors. In order to receive consistent copy quality, the electrical potential should be maintained at a uniform value.
The charge may be delivered to the transfer surface through the use of any one of a number of existing methods. One such method is a roller composed of conductive material supplied with a potential and positioned across the transfer surface so that it may be rolled across while in pressure applying contact with the surface of the transfer member opposite the imaged member.
Another such method is a corona discharge device with an electrode supplied with a potential and positioned to move with respect to the transfer member depositing a charge on the surface of the transfer member opposite the toner image. Still other methods are readily known in the prior art.
The value of the electrical potential applied to the transfer device resulting in a nondistorted transfer has been found to depend on the material of which the transfer surface is made and, more critically, on the relative humidity of the surroundings. Therefore, when a change occurs in any of the variables, the potential used in that transfer must be adjusted.
While readjusting the potential after changes in the transfer material and humidity, the current at which the transfer occurs was discovered as remaining constant regardless of the adjustment in potential.